Various mechanical operations for machining of a large number of workpieces can involve the use in succession, for the same workpiece, of several tools in a programmed and automatically controlled sequence.
For this purpose, the workpiece is usually mounted on a lathe or similar machining center which is provided with an auxiliary apparatus consisting of a turret carrying several tools which are arranged radially and/or frontally on a rotating plate, the controlled rotation of which brings the tool, pre-selected in each case, into the working position for carrying out the required machining of the workpiece.
A fundamental requirement which arises during such machining operations is the reduction in the idle time in particular due to the time required for changing the tool and resuming machining of the workpiece.
This idle time referred to herein as "cycle time" is substantial which is necessary for allowing the turret to perform all the movements designed to bring the new tool into the working position.
For this purpose, in fact, the turret requires a first "opening phase" for releasing itself from the fixed working position, a "rotating phase" for rotating in either direction, along the shortest path, until the new tool is arranged in the working position, and a "stopping and closing phase" for stopping and returning into the locked position.
These operations are performed, in the known turrets, using mechanical devices which transmit the rotation of an actuating motor to the rotating tool-carrying plate and perform at the same time actuation of the locking and release means which comprise, among other things, an index disc provided with radial seats which define the different positions which can be assumed by the tool-carrying plate, and two toothed rims, one fixed to the structure and one rotating with the tool-carrying plate, but axially fixed. The rims are locked together or released during the workpiece machining or tool changing rotation phases by means of a third toothed rim facing the other two, which, moving in an axial direction engages or disengages its own toothing with or from the toothing of the other two rims, causing relative locking or releasing thereof.
It is also known that, during the turret closing phase, the teeth of the two rims to be locked are not perfectly aligned since the devices of the traditional type provided inside the turret for stopping the rotating rim have a certain amount of play necessary for correct operation thereof in order to stop rotation of the tool-carrying plate.
Consequently, front coupling between the toothing of the locking rim and the toothing of the facing rims causes rotational driving of the rotating rim, which terminates when the locking rim is totally engaged on the fixed rim. Correspondingly, engagement of the rotating rim against the teeth of the stopping rim causes an impact which is transmitted to the structure, generating noise and the possibility of damage to the turret.